JP2024044109A - How to recycle waste plastic - Google Patents

Abstract

[Problem] The present invention provides a method for easily and efficiently pyrolyzing waste plastics and recycling them into high-quality pyrolytic wax that is useful for applications such as molding aids for plastics and rubber, lubricants, mold release agents, ink and paint additives, pigment dispersants, hot melt adhesives, etc. [Solution] The method involves feeding waste plastics to an extruder and pyrolyzing them in the extruder, thereby recycling them into pyrolytic wax. [Selected Figure] None

Description

The present invention relates to a method for recycling waste plastic.

Due to its usefulness, plastics are used in a wide range of products and containers and packaging. Meanwhile, the importance of promoting the circulation of plastics is increasing amid calls for the realization of carbon neutrality and for stronger responses to the problems of marine plastic waste and climate change. Chemical recycling, which uses naphtha synthesized by pyrolysis of waste plastic (hereinafter sometimes abbreviated as waste plastic) as a raw material to produce polyethylene (PE) and polypropylene (PP), etc., using naphtha crackers and polymerization plants, has the advantage that even degraded waste plastic can be recycled, and because plastic is synthesized from monomer and oligomer raw materials, recycled products have the same quality as virgin products. However, the recycling process is long and complex, and issues have been pointed out that it is not economically rational.

Here, low-molecular-weight polymers with a molecular weight of 10,000 or less exhibit different physical and chemical properties from general polymers with molecular weights of tens of thousands to hundreds of thousands. In particular, polyolefin waxes such as low-molecular-weight polyethylene and low-molecular-weight polypropylene are highly compatible not only with polyolefins, which account for the majority of plastics produced, but also with resins such as polyvinyl chloride, and are used in a wide range of applications, such as pigment dispersants, molding processing aids, additives for inks and paints, and additives for hot melt adhesives.

There are two methods for synthesizing polyolefin wax: polymerization and pyrolysis. Thermally degraded low molecular weight polyolefins have been proposed (see, for example, Patent Document 1). The pyrolysis method has commercial advantages, such as lower production costs and suitability for small-scale production compared to polymerization, and is also useful from the viewpoint of recycling, which has been given great importance in recent years, as it allows the use of discarded plastics. However, polyolefins have a higher pyrolysis temperature than other polymers, and in order to obtain wax, they must be pyrolyzed at high temperatures for a long period of time, which requires a lot of energy in the pyrolysis process. For this reason, several methods have been proposed to promote the pyrolysis reaction, such as a method of adding oxygen and peroxide (see, for example, Patent Document 2), a method of adding a mercaptan-based organic compound (see, for example, Patent Document 3), and a method of reacting with a contact catalyst (see, for example, Patent Document 4). In addition, a method for producing pyrolysis wax (see, for example, Patent Document 5) has been proposed, which is characterized by supplying an olefin-based polymer to a pyrolysis device composed of a pyrolysis reactor consisting of a quantitative transport means for quantitatively transporting the polymer and a screw-type extruder connected to the quantitative transport means, which are connected in multiple stages. Furthermore, a continuous waste plastic oil reduction device (e.g., Patent Document 6) and a twin-screw extrusion type waste plastic processing method (e.g., Patent Document 7) have been proposed.

Japanese Patent Application Publication No. 10-158329 Japanese Patent Publication No. 51-48196 Japanese Patent Application Laid-Open No. 9-40801 JP 2019-515060 A Japanese Patent Application Laid-Open No. 4-304205 WO2019/004462 publication JP 2008-302555 A

In the low molecular weight polyolefin obtained by thermally degrading polyolefin in a tubular reactor proposed in Patent Document 1, there was a problem that thermally degraded products generated on the inner surface of the reactor were mixed in, resulting in a low molecular weight polyolefin of degraded quality. Furthermore, in the polypropylene obtained by the method proposed in Patent Document 2, the decomposition of polypropylene is accelerated by oxygen and peroxide, so that polypropylene is easily oxidized, and the inherent characteristic of polypropylene, such as low polarity, is impaired. In addition, in the oily matter obtained by the method proposed in Patent Document 3, a mercaptan organic compound is used as a decomposition accelerator in the thermal decomposition of synthetic polymers, so the harmfulness to the human body and odor caused by the residual mercaptan organic compound became a problem. Furthermore, in the wax obtained by the method proposed in Patent Document 4, a contact catalyst is used to convert plastic into wax, so there were problems with the increase in cost due to the use of an expensive catalyst and the deterioration of color due to the inclusion of catalyst residues. In addition, the method proposed in Patent Document 5 can obtain a highly decomposed pyrolysis wax, but there is no mention of the use of waste plastics. Furthermore, the plastic oil reduction device proposed in Patent Document 6 does not state anything about the characteristics of the resulting oil or the pyrolysis wax. Furthermore, the twin-screw extrusion type waste plastic processing method proposed in Patent Document 7 is a method for obtaining molded products including pellets, and does not state anything about pyrolysis wax.

Therefore, an object of the present invention is to provide a method for simply and efficiently pyrolyzing waste plastic, particularly discarded polyethylene resin, and recycling it as high-quality pyrolytic wax. More specifically, the present invention provides a method for recycling pyrolytic waxes, which are useful as molding aids for plastics and rubber, lubricants, mold release agents, ink and paint additives, pigment dispersants, hot melt adhesive applications, etc. There is a particular thing.

Improving the reuse efficiency of molded and used resins and molded objects, such as reusing, recycling, turning them into raw materials, and turning them into oil, will contribute to the promotion of inclusive and sustainable industrialization. It is one of the technologies necessary for a sustainable society such as the SDGs that have been talked about in recent years.

As a result of intensive studies to solve the above problem, the present inventor discovered that it is possible to easily and efficiently recycle waste plastic into high-quality pyrolyzed wax by feeding it to an extruder and thermally decomposing it within the extruder. , we have completed the present invention.

That is, each aspect of the present invention is [1] to [6] shown below.
[1] A recycling method in which waste plastic is fed into an extruder and thermally decomposed in the extruder to produce pyrolytic wax.
[2] The recycling method according to [1], wherein the waste plastic is a polyolefin resin.
[3] The recycling method according to [2], wherein the polyolefin resin is a polyethylene resin.
[4] The recycling method according to any one of [1] to [3], wherein the extruder is a multi-screw extruder having 2 or more screws and 8 screws or less.
[5] The recycling method according to any one of [1] to [3], wherein the extruder is a tandem extruder.
[6] The waste plastic is pyrolyzed in the extruder under the conditions that the cylinder temperature in the pyrolysis region of the extruder is 330 to 480°C, and the tip temperature of the extruder that extrudes the pyrolyzed wax is 100 to 300°C. [1 ] to [5].

The present invention provides a method for easily and efficiently pyrolyzing waste plastics and recycling them into high-quality pyrolytic wax that is useful for applications such as molding aids for plastics and rubber, lubricants, release agents, ink and paint additives, pigment dispersants, and hot melt adhesives, and is of extremely high industrial value.

The present invention will be explained in detail below.

The recycling method of the present invention involves feeding waste plastic to an extruder, where it is thermally decomposed and recycled as pyrolytic wax.

Examples of waste plastics include high-density polyethylene, low-density polyethylene, linear low-density polyethylene, ultra-high molecular weight polyethylene, ethylene-vinyl acetate copolymer (hereinafter sometimes abbreviated as EVA), saponified EVA, and other polyolefin-based resins such as polyethylene and polypropylene, polyester-based resins such as polyethylene terephthalate and polybutylene terephthalate, chlorine-based resins such as polyvinyl chloride, polyamide-based resins such as nylon 6 and nylon 66, styrene-based resins such as polystyrene, and polycarbonate resins, and may also be mixtures of these waste plastics. The form of the plastic is not limited to pellets, powder, etc., and may be molded products such as films, sheets, bottles, fibers, pipes, and injection molded products, as well as crushed products. Examples of waste plastics that may be referred to as used products, non-standard products, and discarded products include waste plastics. When the waste plastic is a resin composition, the components may be mixed together, or may be physically mixed in the form of solids such as pellets or scraps. Among these waste plastics, polyethylene resins containing polyethylene resin as the main component are preferred, as they can be used in a wide range of applications, such as as pigment dispersants, molding processing aids, ink or paint additives, and hot melt adhesive additives, and their molecular weight is easy to control. The polyethylene resin content is preferably 70% by weight or more, and more preferably 80% by weight or more, since it becomes a pyrolysis wax with stable quality.

The extruder is not particularly limited, and examples thereof include single-screw extruders, co-rotating twin-screw extruders, counter-rotating twin-screw extruders, multi-screw extruders such as four or eight screws arranged in parallel in the cylinder of the extruder, and tandem extruders in which two or more extruders are connected in series, in which the outlet of one extruder is connected to the inlet of the other extruder and arranged in an L-shape, and in which the outlet of one extruder is connected to the side of the other extruder and arranged in a T-shape. In addition, examples of the extruders constituting the tandem extruder include single-screw extruders and multi-screw extruders with two or more and eight or less screws, and two or more of these extruders can be combined to form a tandem extruder. Among these extruders, it is preferable to use co-rotating twin-screw extruders, multi-screw extruders with four to eight screws, and tandem extruders, because they can produce pyrolysis wax with stable quality and have excellent stability in torque and discharge amount during extrusion.

The extruder used in the recycling method of the present invention is preferably equipped with a vacuum vent port in order to efficiently discharge pyrolyzed low molecular weight gas components outside the extruder. Furthermore, in order to be able to sufficiently pyrolyze waste plastics and obtain pyrolysis wax with stable quality, it is preferable that the ratio (L/D) of the screw length (L) to the screw diameter (D) is 30 or more, and particularly 40 or more.

In addition, in the recycling method of the present invention, the conditions for supplying the wax to an extruder and pyrolyzing it in the extruder can be adjusted as appropriate depending on the raw material waste plastic, the type of extruder, the desired molecular weight of the pyrolytic wax, etc. However, in order to perform thermal decomposition in a short time and to easily suppress the odor of the resulting thermally decomposed wax, for example, the cylinder temperature in the thermal decomposition region should be in the range of 330 to 480°C. The temperature is more preferably 365 to 475°C, particularly preferably 380 to 450°C. Furthermore, the thermal decomposition time in the thermal decomposition region, that is, the residence time of the polyethylene resin in the extruder, can be in the range of 1 to 30 minutes, more preferably 2 to 20 minutes, and particularly preferably 3 to 30 minutes. ~15 minutes. Furthermore, the temperature at the tip of the extruder when extruding the wax after pyrolysis can be in the range of 100 to 300°C or less, since it is easier to control the molecular weight more precisely; It is more preferable that the temperature be within the range of 100 to 250°C, particularly preferably 100 to 250°C. In addition, in order to suppress the odor of the resulting pyrolyzed wax and to easily control its molecular weight, the interior of the extruder during pyrolysis is purged with an inert gas such as hydrogen, helium, argon, nitrogen, or carbon dioxide. It is preferable that the gas be replaced with nitrogen gas, and it is particularly preferable that the gas be replaced with nitrogen gas. The pyrolyzed wax extruded from the extruder can be made into pellets by hot cutting, mist cutting, underwater cutting, or by cutting after cooling on a steel belt.

EXAMPLES The present invention will be specifically explained below with reference to Examples, but the present invention is not limited thereto in any way.

～GPC measurement of pyrolytic wax～
Measurements were performed using the following measuring devices and instruments under the following conditions.

Equipment: HLC-8321GPC/HT (Detector: RI method) (manufactured by Tosoh Corporation)
Column: Use one column (i) below and three columns (ii) in series.
(i)TSKgel guArdColumuH(HR)(30)HT
(7.5mm I.D.) x 7.5cm) (manufactured by Tosoh Corporation) x 1 (ii) TSKgel GMH (HR)-H (20) HT
(7.5 mm I.D.) x 30 cm) (manufactured by Tosoh Corporation) x 3 Eluent: 1,2,4-trichlorobenzene (BHT: 0.05 wt% content)
(Manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.)
Flow rate: 1.0mL/min Injection volume: 0.3mL
Column temperature: 140℃
System temperature: 40℃
Sample concentration: 1mg/mL
Calibration curve: 5th order approximate curve manufactured by Tosoh Corporation using standard polystyrene. However, the molecular weight was determined as PE equivalent molecular weight using Q factor. It was determined that a pyrolytic wax having a number average molecular weight (Mn) of 500 or more and 10,000 or less is a wax of excellent quality that can be used for a wide range of purposes.

Example 1
As the extruder, a co-rotating twin-screw extruder with a screw diameter of 12 mm and a ratio of screw length L to screw diameter D (L/D) of 85 was used. In addition, two vacuum vent ports were installed to remove volatile gas from the extruder (extruder A-1). Then, as waste plastic, a linear low-density polyethylene film with a number average molecular weight of 14,000 is pulverized in a pulverizer to reduce its volume, and then supplied to the extruder A-1 together with a constant flow of nitrogen gas. The material was thermally decomposed by melting and kneading at a raw material supply rate of 2 kg/hr under the condition that the cylinder temperature in the kneading zone of the first extruder was heated to 430°C. Next, the melted and kneaded polyethylene resin is extruded onto a steel belt installed in a nitrogen atmosphere under the condition that the cylinder temperature at the tip of the extruder is heated to 180°C, cooled, and then cut into pellets. A polyethylene wax of 1.6 kg/hr was obtained at a rate of 1.6 kg/hr. As a result of GPC measurement using the obtained polyethylene wax, the number average molecular weight was 5,000.

Example 2
As the extruder, a tandem extruder was used in which the outlet of the first extruder and the inlet of the second extruder were connected to each other in an L-shape, with a screw diameter of 12 mm and a screw length L to screw diameter D ratio (L/D) of 70 as the first extruder, and a single screw extruder with a screw diameter of 20 mm and L/D of 45 as the second extruder. In addition, in order to remove volatile gases from the extruder, two vacuum vents were installed in the first extruder and one in the second extruder (extruder A-2). Then, as waste plastic, a high-density polyethylene bottle with a number average molecular weight of 20,000 was crushed and reduced in volume in a crusher, and then supplied to the extruder A-2 together with a constant flow rate of nitrogen gas, and pyrolyzed by melt kneading at a raw material supply rate of 2.5 kg/hr under conditions in which the cylinder temperature of the kneading zone of the first extruder was heated to 430 ° C. The melt-kneaded polyethylene resin was then fed to a second extruder connected to the outlet of the first extruder, and extruded onto a steel belt placed in a nitrogen atmosphere under conditions where the cylinder temperature at the tip of the second extruder was heated to 160° C., cooled, and then cut to obtain pellet-shaped polyethylene wax at a rate of 2 kg/hr. GPC measurement was then performed using the obtained polyethylene wax, and the number average molecular weight was found to be 4,000.

Example 3
As the extruder, a co-rotating four-screw extruder with a screw diameter of 12 mm and a ratio of screw length L to screw diameter D (L/D) of 40 was used. In addition, two vacuum vent ports were installed to remove volatile gas from the extruder (extruder A-3). Then, as waste plastic, a low-density polyethylene film with a number average molecular weight of 12,000 is crushed in a crusher to reduce its volume, and then supplied to the extruder A-3 together with a constant flow of nitrogen gas. The material was thermally decomposed by melting and kneading at a raw material supply rate of 3 kg/hr under the condition that the cylinder temperature in the kneading zone was heated to 400°C. Next, the melted and kneaded polyethylene resin is extruded onto a steel belt placed in a nitrogen atmosphere under the condition that the cylinder temperature at the tip of the extruder is heated to 200°C, cooled, and then cut into pellets. 2.5 kg/hr of polyethylene wax was obtained. Then, as a result of performing GPC measurement using the obtained polyethylene wax, the number average molecular weight was found to be 3,000.

Comparative Example 1
A linear low-density polyethylene film having a number-average molecular weight of 14,000 was used as the waste plastic, and was pulverized and reduced in volume by a pulverizer. Then, 2 kg of the pulverized waste plastic was supplied together with a constant flow rate of nitrogen gas into a 15-liter autoclave equipped with a stirrer, and the inside of the autoclave was stirred. Then, the autoclave was heated from room temperature at a rate of 10°C/min, and after reaching 420°C, it was held for 90 minutes. Then, the autoclave was cooled for 120 minutes while supplying nitrogen gas into the autoclave, and 1.5 kg of polyethylene wax was extracted from the autoclave when it reached 140°C. Then, GPC measurement was performed using the obtained polyethylene wax, and the number-average molecular weight was 1,500. The time required to obtain 1.5 kg of the polyethylene wax was 280 minutes.

Comparative example 2
A high-density polyethylene bottle with a number average molecular weight of 20,000 was used as the waste plastic, and the bottle was crushed and reduced in volume using a crusher. Next, 2.5 kg of the resulting pulverized waste plastic was supplied together with a constant flow of nitrogen gas into a 15-liter autoclave equipped with a stirrer, and the inside of the autoclave was stirred. Then, the temperature of the autoclave was raised from room temperature at a rate of 10°C/min, and after reaching 430°C, it was held for 10 minutes. Thereafter, the autoclave was cooled for 60 minutes while supplying nitrogen gas, and when the temperature reached 220°C, 1.8 kg of polyethylene wax was extracted from the autoclave. Then, as a result of GPC measurement using the obtained polyethylene wax, the number average molecular weight was 13,000. The time required to obtain 1.8 kg of the polyethylene wax was 120 minutes.

The present invention thermally decomposes waste plastics simply and efficiently, and is useful for applications such as molding aids, lubricants, mold release agents, ink and paint additives, pigment dispersants, and hot melt adhesives for plastics and rubber. It provides a way to recycle it as a high quality pyrolytic wax.

Claims (6)

A recycling method in which waste plastic is fed into an extruder and thermally decomposed in the extruder to produce pyrolytic wax. The recycling method according to claim 1, wherein the waste plastic is a polyolefin resin. The recycling method according to claim 2, wherein the polyolefin resin is a polyethylene resin. The recycling method according to claim 1, wherein the extruder is a multi-screw extruder having two or more and eight or less screws. The recycling method according to claim 1, wherein the extruder is a tandem extruder. The recycling method according to any one of claims 1 to 5, in which waste plastic is thermally decomposed in an extruder under conditions of a cylinder temperature of 330 to 480°C in the thermal decomposition region of the extruder and a tip temperature of 100 to 300°C for extruding the wax after thermal decomposition.